If lines y=(2a-5)x-5 and 9y=(a+2)x-3 are perpendicular, what are all the possible values of a? Express your answers in decreasing order, separated by commas.
First put both equations in slope-intercept form to express the slope in terms of a.
Then multiply them together and set that product equal to -1 (The product of the slopes of two perpendicular lines is, for the most part, -1)
Then you solve for a using the quadratic formula or factoring.